alimad wrote:
Maths, Physics and chemistry books are stored on a library shelf that can accommodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the probability, that a book selected at random is either a hard cover book or a chemistry book?
A. 1/10
B. 3/20
C. 1/5
D. 1/4
E. 9/20
We know that M = 2P
P = 4 + C
M = 8+2C
8 + 2c + 4 + c + c = 20
4c + 12 = 20
4c = 8
c = 2 , P = 6, M = 12
Please assist further. Thanks
Hello
I am not good at probability, but somehow got this answer right . Could you please explain if the approach used by me is correct or it was just a stroke of luck that i got it right..
Number of Math+Physics+Chemistry books= 25 - ((20/100)*25) = 20
Hence, since M=2P, P=4+C,
(4+C)+C+2(4+C)=20
Therefore, M=12, C=2, P=6
Probability of getting a chemistry book = 2/20 = 1/10
Probability of getting a Hardcover book = P(getting a math or phy book) * P( getting a hardcover book from math or phy books) = (18/20 )*(7/18) = 7/20
Therefore, P(Getting chemistry or hardcover book) = (1/10)+(7/20) = 9/20
I am really doubtful about the red colored approach. Please guide.
Your approach is correct. Though the red part should be {the probability of getting a hardcover